1. Field of the Invention
The present invention relates generally to turbine blades having squealer tips, and more specifically to a turbine blade squealer tip with segments or slots placed in positions of maximum stress.
2. Description of the Prior Art
Combustion turbine power plants, generally, have three main assemblies: a compressor assembly, a combustor assembly, and a turbine assembly. A transition section is located between the combustor assembly and the turbine assembly. In operation, ambient air is drawn into the engine and compressed by the compressor assembly. The compressed air is channeled into the combustor assembly where it is mixed with a fuel, and the compressed air-fuel mixture is then ignited to create a heated working gas. The heated working gas passes through the transition section and into the turbine assembly.
The compressor and turbine stages of the engine include a plurality of turbine blades that are mounted on a common rotating shaft. Each blade is fixed on a rotor disk and extends radially outward with the airfoil to culminate in a tip that is in close proximity to the blade rim. The compressor and turbine stages each additionally include one or more stationary vanes or stators that include non-moving turbine blades and that cooperate with the turbine blades mounted on the rotating shaft to compress air and to derive mechanical power from high velocity gases. Turbine blades and vanes are thus exposed to a high stress environment which can include mechanical, thermal and rotational stressors. In many cases, the thermal stress at the turbine blade tip is the most significant due to the high temperature gradients during transient conditions.
While higher temperatures are required to provide greater engine efficiency, factors which limit the operation temperature include the materials of which the blades and other components are made, and the ability to provide a cooling means to these components. Blades are thus cast to include hollow channels through which a coolant medium can be passed. These hollow channels form the core of the blade. The hollow cavity receives cooling air during operation of the combustion engine to provide a cooling effect to the blades and to control the operating temperature thereof.
The core section of the blade includes an end cap proximate the tip that has a peripheral land encircling it, that on the outer surface forms an extension of the airfoil and on its inner surface defines a groove. The land portion of the airfoil is generally referred to as the squealer tip. Squealer tips are typically found on first and second stage turbine blades, and are generally used on turbine blades to increase efficiency and reduce the loss of hot gas over the tip of the blade. Squealer tips are designed to minimize the distance between the blade and the ring segment surrounding the blade, thus reducing the loss of hot gas.
However, squealer tips are difficult to cool, because cooling air within the interior cavity of the airfoil does not reach the tip. Analysis of currently designed turbine blades shows that the squealer tip is likely to crack during service operation. These cracks are in a radial direction and are slow to propagate. While some radial tip cracking is acceptable, blades are routinely removed from service when the radial tip crack limit is reached. If a crack breaches the internal cooling channel, the cooling air may leak therethrough and adversely affect the intended cooling of the blade.
Due to the high cost of high performance hot section turbine components, it is desirable to repair such components and prolong as much as possible the life of blade tips and the respective blades rather than replace them. Prevention of cracks in the squealer tip can ultimately prevent propagation of these cracks into the airfoil. This is highly desirable, as repair of blades with cracks that extend into the core cavity is very expensive. A variety of methods currently exist for preventing these cracks.
U.S. Pat. No. 5,902,093 discloses an air foil which includes a serpentine cooling circuit extending inside the air foil for channeling the cooling air and thus cooling the blade during operation. The cooling method of this invention is intended to prevent propagation of radial cracks.
U.S. Pat. No. 5,972,424 discloses a method for repairing a gas turbine engine component coated with a thermal barrier coating system. The coating is intended to prevent cracks that develop in the ceramic topcoat from propagating into the blade.
U.S. Pat. No. 5,733,102 and U.S. Pat. No. 5,660,523 relate to a turbine blade squealer tip with an end cap having cooling holes to permit passage of cooling air from the interior of the blade through the end cap. The ""102 patent optionally provides a thermal barrier coating on an outward side of the squealer tip to provide insulation against the combustion gas.
There is a further need to provide suitable methods for prevention of radial cracks in regions of maximum stress in the squealer tip, and to prevent propagation of these cracks into the body of the airfoil.
The present invention solves the above problem by providing stress relief in blade squealer tips in locations that are susceptible to radial tip cracks. The stress relief is provided by means of segments or slots which are machined into the blade during manufacture, in regions that are determined to be of high thermal stress during operation. Such a pre-segmented or slotted squealer tip blade can remain in service for longer periods of time than a similar blade that does not have a pre-segmented or slotted squealer tip. In addition, once a pre-segmented squealer tip blade is submitted for repair, the repair scope will be less extensive than a blade that does not have a pre-segmented squealer tip. The present invention will result in reduced time and costs of gas turbine outages as well as reduced costs and higher yield; more blades can be returned to service after the blade repair process. The present invention also reduces the risk of unacceptable crack propagation into the airfoil during service.
It is an object of the present invention therefore to provide pre-segmented squealer tips in turbine blades which can resist formation and propagation of cracks.
Its is a further object of the present invention to provide pre-segmented squealer tips of turbine blades having slots located in regions of maximum stress.
It is an additional object of the present invention to provide a pre-segmented squealer tip of a turbine blade which reduces the cost of repairing damaged turbine blades.
These and other objects of the present invention will be apparent from the following description, drawings and the appended claims.